In the field of electrochemistry there is a well known electrochemical cell known as a chlor-alkali cell. In this cell, an electric current is passed through a saturated brine (sodium chloride salt) solution to produce chlorine gas and caustic soda (sodium hydroxide). A large portion of the chlorine and caustic soda for the chemical and plastics industries are produced in chlor-alkali cells.
Such cells are divided by a separator into anode and cathode compartments. The separator characteristically can be a substantially hydraulically impermeable membrane, e.g., a hydraulically imperable cation exchange membrane such as the commercially available NAFION.RTM. manufactured by the E. I. du Pont de Nemours & Company. Alternatively, the separator can be a porous diaphragm, e.g., asbestos which can be in the form of vacuum deposited fibers or asbestos paper sheet as are well known in the art. The anode can be a valve metal, e.g., titanium, provided with a precious metal coating to yield what is known in the art as a dimensionally stable anode.
One of the unwanted byproducts present in a chloralkali cell is hydrogen which forms at the cell cathode. This hydrogen increases the power requirement for the overall electrochemical process and eliminating its formation is one of the desired results in chlor-alkali cell operation.
It has been estimated that 25% of the electrical energy required to operate a chlor-alkali cell is utilized due to the formation of hydrogen at the cathode. Hence, the prevention of hydrogen formation at the cathode during the formation of hydroxide, can lead to substantial savings in the cost of electricity required to operate the cell. In fairly recent attempts to achieve cost savings and energy savings in respect of operating chlor-alkai cells, attention has been directed to various forms of what are known as oxygen (air) cathodes. These cathodes prevent the formation of molecular hydrogen at the cathode and instead reduce oxygen to form hydroxyl ions. Savings in cost for electrical energy are thereby achieved.
One known form of oxygen (air) cathode involves use of an active layer containing porous active carbon particles whose activity in promoting the formation of hydroxide may or may not be catalyzed (enhanced) using precious metal catalysts, such as silver, platinum, etc. The active carbon particles may become flooded with the caustic soda thereby significantly reducing their ability to catalyze the reduction of oxygen at the air cathode, resulting in decreased operating efficiency. In an attempt to overcome these difficulties in flooding of the active carbon, hydrophobic materials, e.g., polytetrafluoroethylene (PTFE) have been employed in particulate or fibrillated (greatly attenuated and elongated) form to impart hydrophobicity to the active carbon layer, per se, and/or to a protective (wet proofing) or backing sheet which can be laminated or otherwise attached to the active layer. Thus PTFE has been employed in both active layers and in backing (wetproofing) layers secured thereto. Such active carbon-containing layers, however, are subjected to loss of strength resulting in failure combined with blistering thereof when the chlor-alkali cell is operated at high current densities, viz., current densities ranging from about 250 milliamperes/cm.sup.2 and higher for prolonged time periods.